A scanning system for reducing dead time is described in G.B. Pat. No. 1,419,940. This system comprises two polygons having the same number of facets and rotating on a common axis. One polygon, a prescanner, has convex facets, while the other or main polygon, has flat facets. Laser light is focussed onto a prescanner facet and reflected off a concave mirror back to a main polygon facet. The beam of light on the main facet occupies the whole width of the facet and sweeps round with it as the main polygon rotates. This system is also suggested for use with a detector replacing the laser to form a flying aperture system.
Other known systems use a double reflection off a rotating polygon; the two reflections may be off different ends of the same facet or the polygon may be split into two with two facets behaving as a single facet rotating together. These known systems suffer from dead time because the region where the incident light reflects off the facet remains substantially stationery while the facet rotates.
Most prior art systems operate in what is known as object space, where an image of the scene being swept is focussed far away from the rotating facets. These use a lens system, such as an afocal telescope, so that the facets operate on parallel or near parallel beams of light.